The present invention is generally directed to carbon nanotube—graphene composites, their composition, and their use as temperature sensing elements in devices and articles, especially printable devices and articles of any size including microelectromechanical systems and nanoelectromechanical systems. Graphene is a single planar sheet of sp2-bonded carbon atoms. Parallel-oriented stack of graphene sheets constitutes graphite. A single wall carbon nanotube (SWCNT) is a graphene sheet rolled into a cylinder. A multi wall carbon nanotube (MWCNT) comprises of multiple graphene sheets rolled into concentric cylinders or a graphene sheet rolled into a scroll or multiple graphene sheets rolled into concentric scrolls.
The temperature measurement is a fundamental and ubiquitous necessity. The temperature dependence of electrical resistance of conductive carbons, for instance solid graphite [Bedford & Quinn] has been long known and utilized for fabrication of thermometers functioning well below ambient temperature in the cryogenic range. The thermometric use of graphite has been limited to low range of temperatures because of problems relating to nonunique temperature responses and low resistivity exhibited by these materials near the ambient temperature. Carbon resistors have been used as resistive thermometers as well but their applicability is also limited to low temperatures for reasons of thermal instability, limited range of unique responses and sensitivity. These devices also exhibited nonuniformity of properties relating to composition and thermal treatment history requiring repetitive individual calibration. Carbon-glass resistive sensors exhibit good stability and monotonic change in resistance characteristic between 1.4 K and 325 K, but their reduced sensitivity (0.01 Ohm/K) above 100 K limits their usage at higher temperatures.
In case of nanodevices, the size is an essential feature. The global or remote temperature reading might not accurately reflect potentially present local variation. It is eminently important to measure temperature while using nanosensors, particularly carbon nanotube based sensors as their responses are susceptible to temperature interference. It is requisite for the temperature sensor to be of the similar size as other sensors in a set of sensors or a sensor array. If known, the temperature effect on other nano devices could be compensated for in the device calibration, improving the device's accuracy and reliability.
The need for temperature measurement on that scale is well appreciated yet, serious practical difficulties persist. Metal nanowires are used for temperature mapping at low (cryogenic) temperatures [Nalwa]. However, the nanowires exhibit positive temperature sensitivity coefficient and their stability is questionable. Recently gallium filled CNT thermometer [Gallium] has been developed for temperature range from 50° C. to 500° C. As the melting point of gallium is at about 29.78° C., gallium nanothermometer is inapplicable to bionanosensors. Another serious drawback of this thermometer is the use of transmission electron microscope for readout precluding portability of the device and severely limiting its affordability. Operating in narrower T range light emitting nanothermometer has been demonstrated by Lee, Kotov and Govorov [Lee 2005], but it is inapplicable to measuring temperatures of hidden from view objects.
The use of carbon-based inks is common in the manufacture of printed electronics, for example printed circuit boards or electrodes for sensors. In general, carbon-based ink is a composite material containing a carbon particulate such as graphite, amorphous carbon or a fullerene, suspended in a binder and a solvent. These composite materials are applied to a surface via a number of deposition techniques, and then cured that is allowed to dry, or are subjected to accelerating or enhancing curing treatment. Conductivity enhancing curing usually consists of heat treatment from 50° C. to several hundred degrees Celsius. Non-thermal curing has also been demonstrated [Kirkor]. The curing step is necessary to attain high conductivity in the resulting carbon composites. For thermal curing, the material's conductivity is temperature dependent.
On any size scale an unmanaged temperature dependence of conductivity of carbon based circuits integrated into functional blocks and applications can limit the usefulness of finished products.
Materials with unique temperature signatures, operational ranges higher than carbon-glass composites and that are compatible with printable electronics are needed. Use of graphenic carbon nanoparticles in a conductive carbon composite allows for scaling down the dimensions of the device as well as biological and chemical compatibility. An example here is the expanding presence of carbon nanotube sensors within the growing field of sensors and sensor arrays in the whole range of sizes present. In this type of sensor, a reagent specific to a given analyte is carried by a carbon conductor (E.g., Carbon Nanotube or an ensemble of Carbon Nanotubes) to make a sensing element specific to the analyte of interest. Typically, the analytical response is temperature dependent. Without temperature compensation, such devices are limited to operation in a narrow temperature range.
In the field, sensors are often subjected to temperature changes. Common temperature changes occur in the range from −80° C. to near 100° C., the most frequently measured temperature range. It is thus advantageous to measure temperature in that range and also on a similar scale as that of the size of the sensor.
The use of individual carbon nanotubes as thermometers could be possible with individual calibration. However, electrical properties of carbon nanotubes vary dependent on their internal structure and derivatization [Avouris, Gruner] rendering individual calibration so cumbersome and costly that it is impracticable. Individual cohesive bundles of parallel MWCNT under high vacuum exhibit monotonic dependence of conductivity on temperature in range from 100 to 800 K [Zhou 2004]. Similar behavior was reported for isolated graphite-metal contacts [Shklyarevskii 2005]. The electrical resistance of ensembles of carbon nanotubes depends on chemical exposure often leading to disparately different and often non-monotonic temperature dependence of the electrical properties. The phenomenon is evident by comparison of results from numerous researchers [Kaiser 1998], [Hecht 2006]. Given observed variability of conductivity of CNT containing materials, properties of individual components can not be expected of intermixed composite materials. Thus, carbon nanotubes typically have not been used for temperature sensors.